Contactless ignition system

ABSTRACT

AN IGNITION SYSTEM FOR ALTERNATELY FIRING TWO SPARK PLUGS IN AN INTERNAL COMBUSTION ENGINE. A ROTOR IS USED TO MODULATE THE OUTPUT SIGNAL OF AN OSCILLATOR, WHICH IN TURN TRIGGERS A BISTABLE DEMODULATOR TO PRODUCE A SQUARE WAVE SYNCHRONIZED WITH THE ENGINE. THE SQUARE WAVE IS USED TO ALTERNATELY FIRE TWO SILICON CONTROLLED RECTIFIERS FOR ALTERNATELY DISCHARGING A STORAGE CAPICITOR THROUGH THE PRIMARY WINDINGS OF TWO IGNITION TRANSFORMERS, ELIMINATING THE NEED   FOR A HIGH VOLTAGE DISTRIBUTOR. A CHARGING CIRCUIT IS TRIGGERED TO RECHARGE THE CAPICITOR EACH TIME IT IS DISCHARGED. THE MAXIMUM RATE AT WHICH THE CHARGING CIRCUIT CAN OPERATE IS LIMITED, THEREBY ACTING AS A GOVERNOR TO LIMIT MAXIMUM ENGINE SPEED.

Sept. 20, 1971 J HARmN ETAL 3,605,714

CONTACTLESS IGNITION SYSTEM Filed June 11, 1969 2 Sheets-Sheet l I8 IGfI9 SWITCHING PULSE SPARK CAPACITOR CIRCUIT-1 TRANSFORMER PLUG-1DISCHARGE l7 CIRCUIT r SWITCHING PULSE SPARK CIRCUIT-2 TRANSFORMER lPLUG-2 5 L20 7 TRIGGER TRANSFORMER 2| SENSING DC-TO-DC BISTABLECONDUCTOR n 'r" VOLTAGE DEMODULATOR DETECTOR I I I I CONVERTER(OSCILLATOR) I I ROTOR SE'IESISSR IIIIII i v OUTPUT A IUUUUU IWVWVWWBISTABLE DEMODULATOR OUTPUT B DISCHARGE CAPACITOR VOLTAGE C SWITCH-ITRIGGER VOLTAGE D SWITCH-2 TRIGGER 0 v A VOLTAGE E INVENTORS. JAMES T.HARDIN WILLIAM J. ROBERTS Y MAXIMILLIAN KUSZ ATTORNEY Sept. 20, 1971 J.T. HARDIN ETAL CONTACTLESS IGNITION SYSTEM 2 Sheets-Sheet 2 Filed June11, 1969 INVENTORS. HARDIN J. ROBERTS AXIMILLIAN KUSZ ATTORNEY BY MUnited States Patent O 3,605,714 CONTACTLESS IGNITION SYSTEM James T.Hardin, Lambertville, Mich., and William J. Roberts and MaximillianKusz, Toledo, Ohio, assignors to Eltra Corporation, Toledo, Ohio FiledJune 11, 1969, Ser. No. 832,217 Int. Cl. F02p 3/02 US. Cl. 123-14815 7Claims ABSTRACT OF THE DISCLOSURE An ignition system for alternatelyfiring two spark plugs in an internal combustion engine. A rotor is usedto modulate the output signal of an oscillator, which in turn triggers abistable demodulator to produce a square wave synchronized with theengine. The square wave is used to alternately fire two siliconcontrolled rectifiers for alternately discharging a storage capacitorthrough the primary windings of two ignition transformers, eliminatingthe need for a high voltage distributor. A charging circuit is triggeredto recharge the capacitor each time it is discharged. The maximum rateat which the charging circuit can operate is limited, thereby acting asa governor to limit maximum engine speed.

BACKGROUND OF THE INVENTION This invention relates to an ignition systemfor internal combustion engines having at least two spark plugs whichare alternately fired and in which no mechanical breaker contacts orpoints are required for synchronizing the firing with the rotatingengine.

Conventional ignition systems for internal combustion engines typicallyinclude a pair of mechanical breaker contacts or points which controlthe current flow to a primary winding of an ignition coil where energyis stored, which, upon release, induces a high voltage current in asecondary winding of the ignition coil which is then directed to thespark plugs through a rotating switch commonly called a distributor.Inherent disadvantages in a mechanical breaker system are the mechanicaland electrical erosive forces which cause the points to wear out or topit due to the inductive kick back voltage from the primary of theignition coil. Also, the points may become fouled or coated by filmswhich interfere with their function of making and breaking theelectrical circuit. These problems are further aggravated when theengine is used for marine purposes where the breaker points aresubjected to moisture and corrosive forces. Another inherent problempresent in mechanical breaker point systems is the presence of contactbounce due to mechanical resonance of the moving parts at high enginespeeds. Likewise, electrical erosion and Wear and a high degree ofsusceptibility to environmental conditions cause the conventional highvoltage distributor to be a problem.

SUMMARY OF THE INVENTION The instant invention is directed to a solidstate ignition system for alternately firing at least two spark plugs inan internal combustion engine. A sensing coil is mounted on the engineadjacent to a rotor having electrically conductive and non-conductiveportions. The sensing coil is effective to amplitude modulate the outputof an oscillator between alternately high and low levels as the enginealternately drives the non-conductive and conductive portions of therotor past the sensing coil. A bistable demodulator is triggered by theamplitude modulated output of the oscillator to produce a square wavewhich is synchronized with the engine. The square wave output of thebistable demodulator is applied to the primary winding of a triggertransformer for alternately firing first and second silicon controlledrectifiers. A storage capacitor is connected in a first closed seriescircuit with the first silicon controlled rectifier and the primarywinding of a first ignition coil and in a second closed series circuitwith the second silicon controlled rectifier and the primary winding ofa second ignition coil. The secondary windings of the ignition coils areconnected to fire different spark plugs without the use of a highvoltage distributor.

When the square wave output of the bistable demodulator changes from afirst voltage to a second voltage, the trigger transformer applies apositive triggering signal to the control electrode of one of thesilicon controlled rectifiers, thereby discharging the storage capacitorthrough the primary winding of one of the ignition coils. Again, whenthe bistable demodulator output changes from the second voltage back tothe first voltage, a trigger transformer applies a positive triggersignal to the control electrode of the other silicon controlledrectifier, thereby discharging the storage capacitor through the primarywinding of the other ignition transformer. A DC-to-DC regulated voltageconverter is provided to charge the storage capacitor. The voltageconverter is triggered to recharge the storage capacitor each time oneof the silicon controlled rectifiers is triggered to discharge thecapacitor. The maximum operating rate of the voltage converter isselectably limited, thereby fixing a maximum speed for the internalcombustion engine.

Accordingly, it is the primary object of this invention to provide animproved contactless distributorless ignition system for alternatelyfiring two spark plugs in an internal combustion engine.

It is a further object of this invention to provide an improved methodof limiting the maximum operating speed of an internal combustionengine.

Further objects and advantages of the invention will become apparentfrom the following detailed description, reference being made to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing theessential components of the contactless ignition system of thisinvention, and schematically showing the functional relation shipstherein;

FIG. 2 graphically indicates the wave forms which appear at variousindicated points in the block diagram of FIG. 1; and

FIG. 3 consisting of FIGS. 3a and 3b, is a detailed schematic circuitdiagram of a contactless distributorless ignition system constructed inaccordance with the instant invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2, ablock diagram of the contactless ignition system is shown along with thevoltage wave forms at selected points in the block diagram. A sensingcoil 11 is mounted adjacent to a rotor 12 which is driven in timedsynchronism with the crankshaft of an internal combustion engine (notshown) in which at least two spark plugs are mounted. The sensing coil11 is connected to amplitude modulate the output of an oscillator 13.The rotor 12 is shown as a semi-circular conductor mounted to rotate ona shaft, although other rotor configurations may be used in whichalternately conductive and non-conductive portions will be driven pastthe sensing coil 11. As shown in graph A of FIG. 2, the oscillator willhave an oscillatory output with a controlled low amplitude when thesensing coil 11 senses the presence of the conductive portion of therotor 12 and will have a higher voltage oscillatory output when thesensing coil 11 does not sense the presence of the conductive portion ofthe rotor 12. A bistable demodulator 14 having a time constant greaterthan the oscillatory period of the oscillator 13 and insensitive to thecontrolled low amplitude output is triggered by the amplitude modulatedoutput of the oscillator 13 to generate a square wave which will besynchronized with the engine driven rotor 12, as seen in graph B of FIG.2. The square 'wave output of the bistable demodulator 14 is thencoupled to the primary winding of a trigger transformer 15.

The high voltage for alternately firing the two spark plugs is producedwhen the trigger transformer 15 alternately energizes a first switchingcircuit 16 and a second switching circuit 17 to alternately dischargeenergy stored in a capacitor discharge circuit 18 to a pulse transformer19 and a pulse transformer 20. A 'DC-to-DC regulated voltage converter21 is triggered to recharge the capacitor discharge circuit 18 inresponse to the alternate operation of the switching circuits 16 and 17.The voltage at the capacitor for several cycles is shown in graph C ofFIG. 2. The DC-to-DC regulated voltage converter 21 is designed to havea predetermined maximum operating rate to limit the maximum speed of theinternal combustion engine. This is particularly advantageous in marineengines where the load imposed by the water may be suddenly removed.

Referring now to FIG. 3, a detailed schematic circuit diagram of theimproved contactless ignition system is shown. A battery 25 or othersuitable DC. power source is connected to supply a voltage between apositive line 26 and a negative line 27 for operating the ignitionsystem. The oscillator 13 basically comprises a pair of NPN transistors28 and 29 connected across the lines 26 and 27, with the emitter of thetransistor 28 connected through a resistor 30 to the line 27 and theemitter of the transistor 29 connected through a resistor 31 to the line27. Bias voltage is applied from the collector to the base of thetransistor 28 by a voltage divider including two resistors 33 and 34connected in series. The collector circuit of the transistor 28 includesa parallel tuned resonant LC circuit comprising a capacitor 35 and thesensing coil 11. The base of the transistor 29 is connected through acurrent limiting resistor 36 to the collector of the transistor 28 andthe emitter of the transistor 29 is connected through a series capacitor37 and resistor 38 to the emitter of the transistor 28 to form anin-phase feedback circuit with the transistor 29 operating as an emitterfollower and the transistor 28 operating in essentially a common baseconfiguration. A capacitor 39 is connected to provide a low impedanceA.C. path from the common connection betwen the resistor 32, theresistor 33 and the LC circuit to the line 27.

If nothing more is added, the oscillator, including the transistors 28and 29, can oscillate at the resonant frequency of the parallel tuned LCcircuit including the sensing coil 11 and the capacitor 35, whenever theQ (the ratio of the stored energy to the dissipated energy) of the LCcircuit is such that the closed loop gain of the oscillator is equal toor larger than unity. When the rotor 12 is positioned such that noconductors are within the magnetic field of the sensing coil 11, the Qof the LC circuit is such that the closed loop gain of the oscillator isequal to or larger than unity. When, on the other hand, the rotor 12 ispositioned such that a conductive portion of the rotor 12 is adjacent tothe sensing coil 11, the Q of the LC circuit is such that the closedloop gain of the oscillator is less than unity and the oscillator willnot oscillate. With this arrangement, the oscillator depends on randomdisturbances for starting when the conductive portion of the rotor 12moves away from the magnetic field of the sensing coil 11.

The capacitor discharge portion of the ignition system is triggered inresponse to the envelope of the oscillations. If the oscillator isdependent upon random disturb ances for starting, the relationshipbetween rotor position and oscillator amplitude will be less precise forincreasing amplitude than for decreasing amplitude. Therefore, asupplementary feedback system, including a NPN transistor 42 and a PNPtransistor 43, is connected in parallel with the normal feedback loop toprevent the oscillator output from decreasing below a predeterminedminimum value and thereby eliminate the dependency upon randomdisturbances for ignition timing information when the oscillator outputis increasing. With the supplementary feedback system, the rotor 12 willamplitude modulate the oscillator output between maximum and minimumlevels as shown in graph A of FIG. 2. The demodulator is set soswitching takes place at a level above the minimum and below the maximumoscillator amplitude.

The supplementary feedback system is connected in parallel with thefeedback path which includes the series capacitor 37 and resistor 38between the oscillator transistors 28 and 29. The oscillator output atthe emitter of the transistor 29 is applied through a capacitor 44 and aseries resistor 45 to the bases of the transistors 42 and 43. A resistor46 is connected from between the capacitor 44 and the resistor 45 to thenegative line 27. The emitter of the NPN transistor 42 and the collectorof the PNP transistor 43 are connected to the negative line 27, whilethe collector of the transistor 42 and the emitter of the transistor 43are connected together through a parallel connected DC. bias resistor-47 and A.C. coupling capacitor 48 to the emitter of the transistor 28.

The bases of the transistors 42 and 43 are driven by an A.C. signalwithout a DC. component, supplied through the capacitor 44 from theoscillator output. A.C. voltages which appear across the transistors 42and 43 are applied through the capacitor 48 to the emitter of thetransistor 28 along with the normal positive feedback signal through thecapacitor 37 and the series resistor 38. If the PNP transistor 43 werealone, it would conduct for negative half cycles of the oscillatoroutput. The A.C. component of the voltage across the transistor 43 wouldbe in phase with the oscillator output, and the supplementary feedbacksystem would increase the total feedback. Similarly, if the NPNtransistor 42 were alone, the transistor 42 would produce anout-of-phase feedback signal. When the oscillator output amplitude isabove a minimum design level, the voltage across the transistors 42 and43 is very low, and there is essentially no supplementary feedback. Whenthe oscillator output amplitude is below the design level, the reducedbase drive through the capacitor 44 allows the DO voltage across thetransistors 42 and 43 to increase. The DC. voltage at the emitter of thePNP transistor 43 adds to the base drive for the transistor 43, whilethe base drive for the transistor 42 is unaffected by the DC. level atthe emitter of transistor 43. The resulting unbalance gives a large netin-phase feedback signal whenever the oscillator amplitude decreases toa level where the NPN transistor 42 is not saturated. The emitter-basejunction of the NPN transistor 42 acts as a reference element andprevents the oscillator amplitude from going below this level.Therefore, as the rotor 12 turns, the oscillator amplitude variesbetween a maximum level determined by the supply voltage as applied onthe line 26 and a minimum level determined by the supplementary feedbacknetwork independently of the supply voltage.

The output of the oscillator, taken at the emitter of the transistor 29,is applied through a capacitor 50 and a voltage divider including aresistor 51 and a resistor 52 to the base of a transistor 53. Thetransistor 53 detects, inverts, amplifies, and clips the oscillatoroutput signal. The collector of the transistor 53 is connected through aresistor 54 to the positive line 26 while the emitter is connected tothe ground line 27. The minimum oscillator output amplitude, asdetermined by the emitter-base characteristics of the transistor 42,varies with temperature in the same way as the amplitude required tobias the transistor 53 on. Therefore, very little attenuation of theoscillator output is needed to keep the transistor 53 off whenever theoscillator output is at its controlled minimum level. When theoscillator output is high, the transistor 53 is driven into saturationduring positive half cycles, and a capacitor 55, connected from thecollector of the transistor 53 to the ground line 27, keeps thecollector voltage low during negative half cycles. The resulting voltageacross the transistor 53 has approximately a rectangular wave form,related inversely to the oscillator output envelope and corresponding tothe shape of the rotor 12.

The voltage appearing across the transistor 53 is applied to thebistable demodulator 14 through a voltage divider comprising a pair ofseries connected resistors 56 and 57. The bistable demodulator 14 isessentially a pair of NPN transistors 58 and 59 connected as a Schmitttrigger. The collector of the transistor 58 is connected through aresistor 60 to the positive line 26, while the base is connected throughthe resistor 57 to the ground line 27 and the emitter is connected incommon with the emitter of the transistor 59 and through a resistor 61to the ground line 27. The collector of the transistor 59 is connectedthrough a resistor 62 to the positive line 26 and the base of thetransistor 59 is driven from the collector of the transistor 58. Theresistors 56 and 57 are selected such that the required voltage acrossthe capacitor 55 and the transistor 53 for switching the transistor 58will be between the ripple voltage appearing on the capacitor 55 whenthe transistor 53 is conducting and the lowest anticipated supplyvoltage appearing on the capacitor 55 when the transistor 53 is cut off.When the transistor 53 is cut off, the transistor 58 will conduct andthe transistor 59 will be cut off. When the transistor 53 startsconducting, the transistors 58 and 59 will rapidly change states. Theresistor 61 is connected to the emitters of the transistors 58 and 59 toprovide positive feedback which decreases the switching time and makesthe circuit insensitive to small changes in the voltage across thetransistor 53.

A capacitor 65 and a primary winding 66 of a trigger transformer 67 areconnected in series from the collector of the transistor 59 to theground line 27. The trigger transformer 67 includes a second primarywinding 68 (the primary windings 66 and 68 may be a single, centertapped winding with the center tap connected to the ground line 27) anda pair of secondary windings 69 and 70. (The windings 66, 68, '69, and70 of the trigger transformer 67 are shown connected by a dashed line.)The secondary windings 69 and 70 may also be a single center tappedwinding. When the transistor 59 is cut off, the capacitor 65 will becharged through the resistor 62 and the trigger transformer primarywinding 66 to approximately the supply voltage. When the transistor 69conducts, the capacitor 65 will be rapidly discharged through theprimary winding 66 to produce a first trigger signal.

The base of an NPN transistor 71 is connected through a resistor 72 tothe junction between the capacitor 65 and the primary winding 66. Thecollector of the transistor 71 is connected through a resistor 73 to thepositive line 26 and the emitter is connected to the ground line 27. Acapacitor 74 is connected in series with the second trigger transformerprimary winding 68 and a parallel resistor 75 between the collector ofthe transistor 71 and the ground line 27. The capacitor 74 will becharged to approximately the supply voltage on the line 26 when thetransistor 71 is cut off. When the transistor 59 is ofi and thecapacitor 65 starts charging, the impedance of the primary winding 66 islarge and the transistor 71 is switched on. The capacitor 74 is thenrapidly discharged through the transistor 71 and the primary winding 68to produce a second trigger signal. The polarities of the two primarywindings 66 and 68 are such that the discharge of the capacitor 74through the primary winding 68 provides a positive feedback in theprimary winding 66 to help saturate the transistor 71, with the baseresistor 72 preventing the emitter-base junction of the transistor 71from limiting the primary winding voltages.

The first and second trigger signals, produced by discharging thecapacitor 65 through the trigger transformer primary 66 and bydischarging the capacitor 74 through the trigger transformer primarywinding 68, respectively, are used to alternately trigger the dischargeof energy stored in a capacitor 76 through the primary of a firstignition transformer 77 and the primary of a second ignition transformer78. The anode and cathode of a first silicon controlled rectifier 79 areconnected in a closed series circuit with the capacitor 76 and theprimary of the first ignition transformer 77. The anode and cathode of asecond silicon controlled rectifier and the primary of the secondignition transformer 78 are connected in a second closed series circuitwith the capacitor 76, with the cathodes of the two silicon controlledrectifiers 79 and 80 connected together. The secondary winding 69 of thetrigger transformer 67 is connected between the control electrode andthe cathode of the silicon controlled rectifier 79 and the secondwinding 70 is connected between the control electrode and the cathode ofthe silicon controlled rectifier 80. The polarities of the primary andsecondary windings 66, 68, 69, and 70 of the trigger transformer 67 aresuch that, when the capacitor 65 is discharged through the primarywinding 66, a positive pulse is applied to the control electrode of thesilicon controlled rectifier 79, triggering the silicon controlledrectifier 79 to discharge energy stored in the capacitor 76 through theprimary of the ignition transformer 77 and to thereby produce a highsecondary voltage for firing a first spark plug (not shown). A negativepulse is simultaneously applied to the control electrode of the siliconcontrolled rectifier 80. This negative pulse prevents triggering thesilicon controlled rectifier 80. Similarly, the winding polarities aresuch that when the capacitor 74 is discharged through the primarywinding 68, a positive trigger pulse is applied to the control electrodeof the silicon controlled rectifier 80 and a negative pulse is appliedto the control electrode of the silicon controlled rectifier 79. Whenthe silicon controlled rectifier 80 is triggered, energy stored in thecapacitor76 is discharged through the primary of the ignitiontransformer 78, providing a high secondary voltage for firing a secondspark plug (not shown). A diode 81 and a resistor 82 are placed inparallel with the capacitor 76, a diode 83 and a resistor 84 are placedin parallel with the primary of the ignition transformer 77, and a diode85 and a resistor 86 are placed in parallel with the primary winding ofthe ignition transformer 78. The diodes 81, 83 and 85 and the resistors82, 84 and 86 protect the silicon controlled rectifiers 79 and 80 fromreverse voltages, prevent reverse charging of the capacitor 76, andprovide discharge paths if the ignition transformers 77 and 78 aredisconnected or open.

Each time the capacitor 76 is discharged to fire a spark plug, theDC-to-DC regulated voltage converter 21 is simultaneously triggered torecharge the capacitor 76. The converter 21 basically comprises a powertransformer 89, a current switching transistor 90, a charging diode 91,and a control circuit for the switching transistor 90. A primary winding92 of the power transformer 89 and the switching transistor 90 areconnected in series across the power lines 26 and 27. When the converter21 is triggered,

charging diode 91. The charging diode 91 prevents the capacitor 76 fromcharging while energy is being stored in the transformer 89 and fromdischarging through the sec-' ondary winding 93 while the capacitor 76is waiting to be discharged through one of the silicon controlledrectifiers.

The control circuit for the switching transistor 90 generally comprisesa modified monostable multivibrator including a normally conductingtransistor 94 and a normally non-conducting transistor 95. The base ofthe tran sistor 94 is connected through a resistor 96 to the positiveline 26 and through a reverse biased diode 97 to the ground line 27. Theemitters of the transistors 94 and '95 are connected to the ground line27. The collector of the transistor 94 and the base of the transistor 95are connected together and are connected through a normallynon-conducting transistor 98 and a resistor 99 to the positive line 26.The collector of the transistor 95 is connected to the base of thecurrent switching transistor 90 and is also connected through a seriesconnected diode 100, a diode 101 and a capacitor 102 to the base of thetransistor 94. A resistor 103 is connected from the positive line 26 toa common point between the capacitor 102 and the diode 101.

When the multivibrator is in the stable state, the transistor 94 is heldon by current flowing through the resistor '96, the transistor 95 isheld 01f, and the capacitor 102 is charged through the resistor 103 toapproximately the voltage of the battery 25. To start the converter 21,a negative trigger pulse is applied through a capacitor 104 to the baseof the conducting transistor 94 and through a series connected capacitor105 and resistor 106 to the base of the non-conducting transistor 98.This negative trigger pulse is applied from the anode of the siliconcontrolled rectifier 79 through a resistor 107, when the siliconcontrolled rectifier 79 is fired. When the silicon controlled rectifier80 is fired, the negative trigger pulse is taken from the collector ofthe transistor 71. At least one of the negative trigger pulses must beindependent of the firing of the silicon controlled rectifiers to permitinitial starting of the converter 21.

When a negative pulse is applied through the capacitor 104 to the baseof the transistor 94, the transistor 94 momentarily loses base currentand turns off. The negative pulse through the capacitor 105simultaneously turns on the transistor 98. Current flow through theresistor 99 and the conducting transistor 98 turns the transistor 95 on,discharging the previously charged capacitor 102 through the diodes 101and 100 and the conducting transistor 95. While the transistor 95remains on, the transistor 98 is held on by base current through thediode 100. As the capacitor 102 discharges, the voltage at the base ofthe transistor 94 is set at a level that is below ground by the forwardvoltage drop of the diode 97. Also, while the transistor 95 isconducting, the voltage of the battery 25 is applied to timingcomponents including the resistor 96 and the capacitor 102. Currentthrough the resistor 96 gradually increases the voltage at the base ofthe transistor 94 from its initial negative value to a positive valueSllffiCiGIlt to turn the transistor 94 back on, returning the system toits stable state.

As previously stated, the time interval during which the currentswitching transistor '90 conducts after the converter 21 is triggered isinversely proportional to the unregulated voltage of the battery 25.This time interval will be accurate if the voltage change on thecapacitor 102 is small compared to the voltage applied by the battery25. When the change in the capacitor 102 voltage is small, the productof the applied voltage and the time increment is approximately equal toa constant. In the instant circuit, the voltage change on the capacitor102 is equal to the forward voltage drop of the diode 97 plus theforward base-to-ernitter voltage of the transistor 94, which issufiiciently small that acceptable regulation can be maintained with asupply voltage as low as four volts.

The current switching transistor 90 is connected to the output of themultivibrator such that the transistor conducts whenever the transistoris conducting. A winding 108 on the transformer 89 along with a seriesresistor 109 is connected to raise the base voltage of the transistor 90slightly above the emitter voltage when the emitter-to-collector voltageis high, to minimize leakage through the transistor 90. The seriesresistor 109 limits the winding current to avoid excessive losses.Therefore, it can be seen that the transistor 90 becomes conductive inresponse to a trigger pulse generated either when the transistor 71 isturned on or when the silicon controlled rectifier 79 is fired. Thetransistor 90 remains conductive for a period of time inverselyproportional to the voltage applied by the battery 25, allowing thecurrent to build up in the primary winding 92 of the transformer 89 to aregulated maximum. When the transistor 90 is switched off by themultivibrator returning to its stable state, the collapsing magneticfield in the transformer 89 produces a high voltage across the secondarywinding 93 which charges the capacitor 76.

To avoid abnormal component stresses resulting from excessive current inthe primary winding 92 of the power transformer 89, a recycle ratelimiter prevents the converter 21 from responding to a trigger pulsewhile current flows from the secondary winding 93 to the capacitor 76.This may be accomplished by connecting the secondary winding 93 toground through the base-emitter junction of the transistor 94, so thatthe transistor 94 cannot turn 011 while there is secondary current flow.If the rotor 12 is turned faster than the maximum recycle rate at whichthe converter 21 will operate, the converter 21 will ignore enoughtrigger pulses to keep its maximum operating rate at a safe level. Justabove the maximum recycle rate, the converter 21 responds only toalternate trigger pulses and the capacitor 76 will be charged for thefiring of only one of the silicon controlled rectifiers 79 and 80.Operation under these conditions will not harm either the ignitionsystem or the engine. The recycle rate limiter may be modified byconnecting the secondary winding 93 directly to the negative line 27 andthen providing an RC time delay circuit to prevent recycling theconverter 21 while there is current flow in the secondary winding 93.

If there is a decrease in the voltage applied by the battery 25 whilethe ignition system is operating, current will flow through thecapacitor 102 away from the base of the transistor 94, which could givea false starting signal. To prevent the converter from starting, thetransistor 98 normally blocks the path through which base current flowsto the transistor 95. The opposite change in supply voltage would tendto trigger the transistor 98, but any change in supply voltage will beignored by either the transistor 94 or the transistor 98 and will notstart the converter 21. After the start of a normal cycle, thetransistor 98 is held on by current flow through the dode 100 and thetransistor 95, so that normal operation is unaffected. A resistor 110,connected between the emitter and the base of the transistor 98,prevents the system from operating at undesirably low voltages. The baseof the transistor 98 is also connected to the converter input throughthe series resistor 106 and capacitor and to the supply voltage line 26through a resistor 111. The diode 100 isolates the transistor 98 fromthe transformer 89. Although the diode 100 and the diode 101 havesimilar functions, separate diodes are used so that the capacitor 102does not slow recovery of the transistor 98.

While the discussion has been limited to engines having two spark plugs,the same system can be used to fire more spark plugs so long as they arefired in two alternate sets. For example, a four-cylinderfour-stroke-cycle engine can often be made to operate with only twoalternate spark plug firing voltages, applied to two spark plugs at atime.

What we claim is:

1. An ignition system for alternately firing two spark plugs in aninternal combustion engine, comprising, in

combination: means for generating a substantially square wave having afrequency proportional to the engine speed, said square wave alternatingbetween first and second voltages, a storage capacitor, means forcharging said storage capacitor, first and second switches, first andsecond ignition transformers, each of said ignition transformers havinga primary winding and a secondary winding, means for connecting saidsecondary windings of said transformers across different ones of suchspark plugs, means connecting said capacitor, said first switch and saidprimary winding of said first transformer in a closed series circuit,means connecting said capacitor, said second switch and said primarywinding of said second transformer in a closed series circuit, means forenergizing said first switch in response to said square wave changingfrom the second voltage to the first voltage whereby said capacitor isdischarged through said primary winding of said first transformer toproduce a high secondary voltage, and means for energizing said secondswitch in response to said square wave changing from the first voltageto the second voltage whereby said capacitor is discharged through saidprimary winding of said second transformer to produce a high secondaryvoltage.

2. An ignition system for alternately firing two spark plugs in aninternal combustion engine, as defined in claim 1, wherein said meansfor charging said storage capacitor is triggered to re-charge saidcapacitor each time one of said first and second switches is energizedto discharge said capacitor.

3. An ignition system for alternately firing two spark plugs in aninternal combustion engine, as defined in claim 2, and including meansfor limiting the maximum repetitive rate at which said means forcharging said capacitor can be triggered to re-charge said capacitor,whereby the maximum speed of the internal combustion engine is limited.

4. An ignition system for alternately firing two spark plugs in aninternal combustion engine, as defined in claim 1, wherein said firstand second switches are siliicon controller rectifiers having anode,cathode and control electrodes, said anode and cathode of said firstsilicon controlled rectifier being connected in series with saidcapacitor and said primary winding of said first transformer, said anodeand cathode of said second silicon controlled rectifier being connectedin series with said capacitor and said primary winding of said secondtransformer, said means for energizing said first switch includes atrigger transformer connected to apply a triggering signal to saidcontrol electrode of said first silicon controlled rectifier, and saidmeans for energizing said second switch includes a trigger transformerconnected to apply a triggering signal to said control electrode of saidsecond silicon controlled rectifier.

5. An ignition system for alternately firing two spark plugs in aninternal combustion engine comprising, in combination: means forgenerating a substantially square wave having a frequency proportionalto the engine speed, said square wave alternating between first andsecond voltages, a storage capacitor, means for charging said storagecapacitor, first and second electronic switches,

said switches having anode, cathode and control electrodes, first andsecond ignition transformers, said transformers having primary andsecondary windings, means for connecting said secondary windings of saidignition transformers across different ones of such spark plugs, meansconnecting said capacitor, said anode and cathode of said first switchand said primary winding of said first transformer in a closed seriescircuit, means connecting said capacitor, said anode and cathode of saidsecond switch and said primary winding of said second transformer in aclosed series circuit, said cathodes of said switches being connectedtogether, a trigger transformer having a primary winding and a centertapped secondary winding, means connecting said center tap to saidcathodes of said switches, means connecting one end of said centertapped secondary winding to said control electrode of said first switch,means connecting the other end of said center tapped secondary windingto said control electrode of said second switch, and means for couplingsaid square wave to the primary winding of said trigger transformerwhereby, when said square wave changes from the second voltage to thefirst voltage, a positive triggering signal is applied to the controlelectrode of said first switch to discharge said capacitor through theprimary winding of said first transformer and, when said square wavechanges from the first voltage to the second voltage, a positivetriggering signal is applied to the control electrode of said secondswitch to discharge said capacitor through the primary winding of saidsecond transformer.

6. An ignition system for alternately firing two spark plugs in aninternal combustion engine, as defined in claim 5, wherein said meansfor charging said storage capacitor is triggered to deliver a regulatedcharge to said capacitor each time said capacitor is discharged, andincluding means for limiting the maximum speed of the internalcombustion engine by limiting the maximum rate at which said chargingmeans can be triggered.

7. An ignition system for alternately firing two spark plugs in aninternal combustion engine, as defined in claim 5, wherein said meansfor generating a square wave having a frequency proportional to theengine speed comprises: an oscillator, means for amplitude modulatingthe output of said oscillator in synchronism with the engine, themodulated output of said oscillator alternating between predeterminedhigh and low levels, and a bistable demodulator, said demodulator havinga square wave output when triggered by the amplitude modulated output ofsaid oscillator.

References Cited UNITED STATES PATENTS 3,242,916 3/1966 Caufal 123-148E3,316,448 4/1967 Hardin et al. 315209 3,356,896 12/1967 Shano 123148E3,361,123 1/1968 Kasama et al 123148E 3,383,556 5/1968 Tarter 315209LAURENCE M. GOODRIDGE, Primary Examiner US. Cl. X.R. 3l5-209

